Chemical-mechanical planarization (hereafter CMP) is a process employed in the fabrication of semiconductors. Silicon wafer substrates containing hundreds of semiconductor devices are brought into contact with a rotating planarization table covered by a polishing pad. Chemicals are added to accelerate and enhance the planarization of the wafer.
The CMP process can be separated into two major categories:
The material transition process involving the complete removal of a material, such as tungsten or copper, from the surface of the wafer until the underlying material layer is exposed,
The film thinning process involving the removal of material, such as SIO2 or silicon, until a predetermined thickness remains.
Optical end pointing is one method of monitoring the reflectivity of the wafer surface during planarization and controlling the CMP process based on changes in said reflectivity. Both CMP categories can be successfully monitored with the use of monochromatic light. However, monochromatic light does not allow instantaneous film thickness measurement for the film thinning process. Endpoint systems using monochromatic light can only infer the amount of film removed during the process by monitoring the process over time, compiling several measurements, and subtracting a known beginning thickness. Endpoint systems using broadband light analyze several wavelengths of the reflectance simultaneously, and can thus measure the instantaneous film thickness directly.
In either case, the key to accurately measuring the wafer surface reflectivity is to position an optical sensor in such a way as to receive a noise-free signal. Sources of noise include thermal variations associated with the CMP process, electro-magnetic interference (EMI), light absorption, lens fogging, electrical slip-ring resistance fluctuations, and air bubbles.
In the case where the sensor is an electronic device, EMI, thermal, and slip-ring noise pose problems. Typically, polishing tables are driven by large variable-speed electric motors, which emit strong electro-magnetic fields of various frequencies. Any electrical conductor transitioning these fields will be subject to induced noise.
Opto electronic devices brought into contact with the CMP process are subject to the thermal fluctuations of the process. Polishing pad friction and exothermic chemical reactions create wide temperature changes. Optical responsivity, Johnson noise, and shot noise all increase as a function of temperature. Therefore, opto-electronic devices in contact with the CMP process typically need to be temperature stabilized before their output is a true measure of the incident radiation. Also, polishing pads are manufactured using heat and pressure. In some cases manufacturing temperatures can exceed the maximum temperature rating specified in the data sheets of the device. Exceeding this rating shortens their life span, or even causes immediate malfunction.
Electrical slip rings used to couple signals from the rotating table are very sensitive to the corrosive CMP chemicals and their vapors. They wear quickly in this environment and begin to suffer from intermittent variations in their contact resistance, which results in random sensor noise. Mercury wetted slip rings are less susceptible this problem, but they are typically limited to operating temperature below 70 degrees Celsius.
Optical sensors installed into the polishing table rely upon a transparent window glued into a hole punched completely through the polishing pad. This arrangement also suffers from optical noise problems, because the pad window tends to leak and thereafter forms a layer of condensate on its under side. Light passing through the condensate layer is scattered, which results in unreliable sensor performance. This particular problem is also temperature related, and can produce indeterminate effects on the optical signal integrity.
CMP processes rely heavily upon liquid chemicals known as slurry, which are not equally translucent to all wavelengths of light. The pad window will carry a layer of slurry along as it transitions under the wafer. As the thickness of this slurry layer increases, more of the light is lost due to absorption and scattering. The thickness of the slurry layer is affected by the position of the pad window with respect to the center of the wafer. For CMP tools whose spindles oscillate and rotate during processing, the slurry layer trapped between the pad window and the wafer will vary as the spindle traverses from side to side. It is theorized that this effect is caused by the difference in relative velocity between the wafer and the polishing pad. At one extreme in the spindle's stroke the wafer is rotating in the same direction as the polishing pad, and at the other extreme the wafer is rotating in the opposite direction. As the relative velocity between wafer surface and pad increases, the slurry layer between pad window and wafer shrinks as the wafer is sucked towards the pad, and the pad window deflects upwards in response to the suction. The result is a periodic disturbance in the optical signal, whose frequency is equivalent to the spindle oscillation frequency, and whose strength is a function of slurry translucence, wafer diameter, table speed, spindle speed, and spindle oscillation stoke.
Air bubbles can produce temporary lens-like occlusions at the pad window and diffract the light in unexpected directions. Even seemingly small bubbles trapped along the fringes of the window can pose a problem when they are sandwiched between the wafer and window lens. The creation of these bubbles increases with table speed and may be due to air being sucked out from the cavity beneath leaking pad windows.
Low-pass filtering is commonly used to attenuate the noise. In most cases, frequency components of the reflectance signals are low compared to those for the noise. Sometimes high order, low cut-off frequency low pass filtering are necessary to adequately attenuate the noise. Material transition processes often exhibit rapid changes in reflectivity at the instant of break through. Using such filtering introduces a significant phase shift in the observed reflectance signal, which causes the endpoint control system to lag behind the process, and results in over-polishing of the wafer.
Most CMP tools do not allow for direct optical inspection of the wafer's surface during the CMP processing, because the wafer is sandwiched between a chuck and the polishing pad. Polishing pads can be manufactured with an integrated optical waveguide to provide a path for indirect optical access. The CMP process can then be monitored by coupling light into this waveguide and measuring the light, reflected by the wafer, emitting from the waveguide. The chemical-mechanical-planarization tool control system may then be tailored to control the process based on the particular type of signal, optical or electronic, provided by the receiving device.